25.3.4 Specifying Options in an Argp Parser

The options field in a struct argp points to a vector of
struct argp_option structures, each of which specifies an option
that the argp parser supports. Multiple entries may be used for a single
option provided it has multiple names. This should be terminated by an
entry with zero in all fields. Note that when using an initialized C
array for options, writing { 0 } is enough to achieve this.

Data Type: struct argp_option

This structure specifies a single option that an argp parser
understands, as well as how to parse and document that option. It has
the following fields:

const char *name

The long name for this option, corresponding to the long option
‘--name’; this field may be zero if this option only
has a short name. To specify multiple names for an option, additional
entries may follow this one, with the OPTION_ALIAS flag
set. See Argp Option Flags.

int key

The integer key provided by the current option to the option parser. If
key has a value that is a printable ASCII character (i.e.,
isascii (key) is true), it also specifies a short
option ‘-char’, where char is the ASCII character
with the code key.

const char *arg

If non-zero, this is the name of an argument associated with this
option, which must be provided (e.g., with the
‘--name=value’ or ‘-charvalue’
syntaxes), unless the OPTION_ARG_OPTIONAL flag (see Argp Option Flags) is set, in which case it may be provided.

int flags

Flags associated with this option, some of which are referred to above.
See Argp Option Flags.

const char *doc

A documentation string for this option, for printing in help messages.

If both the name and key fields are zero, this string
will be printed tabbed left from the normal option column, making it
useful as a group header. This will be the first thing printed in its
group. In this usage, it’s conventional to end the string with a
‘:’ character.

int group

Group identity for this option.

In a long help message, options are sorted alphabetically within each
group, and the groups presented in the order 0, 1, 2, …, n,
-m, …, -2, -1.

Every entry in an options array with this field 0 will inherit the group
number of the previous entry, or zero if it’s the first one. If it’s a
group header with name and key fields both zero, the
previous entry + 1 is the default. Automagic options such as
‘--help’ are put into group -1.

Note that because of C structure initialization rules, this field often
need not be specified, because 0 is the correct value.